1. Field of the Invention
The present invention relates to a method of fabricating a compound semiconductor layer structure and a method of fabricating a device including compound semiconductor crystal.
2. Description of Related Art
Presently, a semiconductor laser of a 1.3 .mu.m band or 1.55 .mu.m band used for optical communications is basically fabricated with InGaAsP/InP-series material. With that material series, when a heterojunction is formed, a leap appearing in its condunction band or a band offset amount .DELTA.E.sub.c is small. Therefore, when its temperature rises, carriers are likely to overflow readily. As a result, its thermal characteristic decreases, and degradation of the device's threshold, efficiency and the like due to the rise in temperature is a serious problem. Recently, that problem has been slightly solved by using AlGaInAs containing Al mixed crystal. That technology, however, is not satisfactory for the purpose of applying to a low-cost laser and the like which require no temperature control.
On the other hand, the development of a blue-color laser using material, into which nitrogen is introduced as V-group element, has been active. That device can also be employed as a long wavelength range laser with a small band gap when mixed crystal, whose content of nitrogen is small, is used. For example, in a single quantum well laser in which a quantum well layer of InGaAsN (its nitrogen content is 0.5%) and barrier layers of AlGaAs are formed on a GaAs substrate, laser oscillation at a wavelength of about 1.2 .mu.m has been reported (see Kondow, et al. Pre-delivered Papers of '96 Spring Meeting of Japan Applied Physics Academy, 27p-C-6). In such a nitrogen-introduced device, since energy of a bottom of its conduction band is greatly lowered from a vacuum level, its band offset amount .DELTA.E.sub.c is quite large and shows a value of about 500 meV that is approximately five times as large as a value of InGaAsP-series. Therefore, in such a device, the thermal characteristic is considerably improved, and there is a possibility that the device shows a practical performance up to high temperatures without requiring any temperature control. Actually, that laser shows its characteristic temperature T.sub.0 =126 K that is about twice as large as a value of an ordinary InP-series laser (see Kondow, et al. Pre-delivered Papers of '96 Autumn Meeting of Japan Applied Physics Academy, 8p-KH-7).
In order to introduce nitrogen into crystal, there exists technology, so-called nitrification, that V-group element in the crystal is substituted by nitrogen by irradiating a substrate surface with nitrogen, other than an ordinary crystal growth technology in which nitrogen is supplied together with other elements during the growth (see Yamamoto, et al. Pre-delivered Papers (separate vol. 1) of Japan Applied Physics Academy, '95 Spring Meeting 28p-ZH-14 and 28p-ZH-16 and '96 Autumn Meeting 9a-ZF-3). That nitrification technology provides a substrate for growing GaN-series crystal thereon, which is primarily used in a blue light emitting device and an electronic device, and thus a GaN layer is formed on a surface of a GaAs substrate by substituting As of the GaAs substrate by N. As the nitrification condition, the substrate temperature is 900.degree. C., a 100% gas of NH.sub.3 is supplied at 3 l/min for ten (10) minutes, and a film with a depth of about 1 .mu.m on the substrate is changed to GaN. Since GaN is formed on the GaAs substrate, a difference in lattice constants therebetween amounts to at least about 20%, and hence its crystal quality is deteriorated. Thus, no single crystal can be obtained.
Further, there is a case where only a very thin film (about 10 nm) on the surface is changed to a GaN layer (see Yao, et al. Pre-delivered Papers (separate vol. 1) of Japan Applied Physics Academy, '96 Autumn Meeting 7a-ZF-2). In that technology, a GaAs substrate is irradiated with nitrogen plasma excited by rf, and its purpose is to improve the quality of the substrate surface but not to control the amount of nitrogen for subsitution. Moreover, that technology aims at making a lattice constant of the surface film close to that of GaN of cubic system.